Because flat batteries such as a button-shaped battery and a coin-shaped battery are small and thin, they are widely used for devices such as a wristwatch and a hearing aid which require reducing the size thereof, and devices such as an IC card which requires reducing the thickness thereof.
FIGS. 17A and 17B show exteriors of a conventional coin-shaped battery. As shown in FIG. 18, a positive electrode pellet 42 and a negative electrode pellet 43 formed into a disk shape are placed opposing to each other with a separator 44 interposed therebetween in a seal case 45 formed as a circular half-shell body. Then, a gasket 46 is attached to an opening of the seal case 45 after electrolyte is filled, and a battery case 41 is placed over the seal case 45 before an open end of the battery case 41 is bent toward the inside for caulking. As a result, the opening of the seal case 45 is sealed to produce this coin-shaped battery 40. The plane shape is circular because the circle is proper for uniformly caulking the openings of the battery case 41 and the seal case 45 along the entire periphery for sealing.
If the plane shape of this flat battery is rectangular, the space efficiency of a battery storage of a device using the battery increases. Moreover, the discharge capacity, and consequently the application range of the flat battery increase because an electrode plate group having a wound structure is easily applied. Japanese Patent Laid-Open Publication No. 2000-164259 discloses a flat battery with a rectangular plane shape.
This flat prismatic battery as shown in FIGS. 19A and 19B is produced by the following steps. That is, a seal case 53 and a battery case 52 formed as a half-shell body with a round rectangular plane shape are placed while their openings are facing to each other, an electrode plate group with a wound structure is stored in an inner space of the seal case 53 and the battery case 52, and the battery case 52 and the seal case 53 are sealed together by shrinking an open end of the battery case 52 for caulking. However, because there is a difference in strength against the caulking between at a rounded corner and at a straight part connecting the corners with each other in the prismatic seal case 53, the straight parts are not surely sealed, and there is such a problem as preventing a leak is difficult.
Namely, when the battery case 52 and the seal case 53 formed as a prismatic half-shell body are placed opposing to each other with a gasket 54 interposed therebetween, and caulking is applied to bend the open end of a side peripheral surface 52a of the battery case 52 on a step 53b formed on a side peripheral surface 53a of the seal case 53 as shown in FIG. 20A, a deformation caused by pressing is not generated at the corners of the seal case 53, and the side peripheral surface 52a of the battery case 52 presses the gasket 54 on the step 53b to complete seal as shown in FIG. 20B. However, since a strength for withstanding the pressure during the caulking is not secured on the straight parts of the side peripheral surface 53a of the seal case 53, a deformation is generated on the step 53b at a location A, the side peripheral surface 53a extends toward the outside at a location B, and a bottom surface of the seal case 53 bulges at a location C as shown in FIG. 20C. When the seal case 53 deforms, the seal is not secured, and this causes a leak.
A double seaming method disclosed in Japanese Patent Laid-Open Publication No. Hei. 6-260172 is widely known as sealing method for preventing the deformation at the straight parts of the seal case. In this flat prismatic battery 60 shown in FIGS. 21 and 22, an electrode plate group 64 with a wound structure is stored in a battery case 61, and a seal plate 62 is provided at an opening of the battery case 61. Then, periphery thereof is seamed twice with a gasket 63 interposed therebetween to improve leak-proof capability. Simultaneously, electrically insulating a positive electrode and a negative electrode of the battery from each other is achieved.
However, there is such a problem as a seamed part 65 largely extends toward the outside of the battery main unit in the sealing using the double seaming as shown in the drawing, and the battery size increases. The purpose of forming the battery in a prismatic shape to increase the space efficiency is therefore not achieved. Also, it is necessary to round the corner of the prismatic case with a large radius for the double seaming, and it is difficult to form the case into a prismatic shape with high space efficiency.
An object of the present invention is to provide a flat prismatic battery with a prismatic shape with high space efficiency and a secure seal structure.